A huge market for LEDs is in replacement lamps for standard, screw-in incandescent light bulbs, commonly referred to as A19 bulbs, or less formally, A-lamps. The letter “A” refers to the general shape of the bulb, including its base, and the number 19 refers to the maximum diameter of the bulb in eighths of an inch (e.g., 2⅜″ diameter). Such a form factor is also specified in ANSI C78-20-2003. Therefore, it is desirable to provide an LED lamp that has the same screw-in base as a standard light bulb and approximately the same size diameter or less. Additional markets exist for replacing other types of standard incandescent bulbs with longer lasting and more energy efficient solid state lamps.
Typical LED lamps having an A-shape use high power LEDs in order to use as few LEDs as possible to achieve the desired lumen output (e.g., 600-1000 lumens). Such LEDs may each draw a current greater than 300 mA and dissipate 1 W or more. Since the LED dies are on the order of about 1 mm2, adequate heat removal is difficult because the heat is usually highly concentrated within a small surface area and often located near the base of the lamp where it shares heat dissipation capacity with the power supply electronics. The high power LED junction temperatures should be kept under 125° C. to ensure the LEDs remain efficient and have a long life. A common design is to mount high power LEDs on a flat, heat conductive substrate and provide a diffusive bulb-shaped envelope around the substrate. The power supply is in the body of the lamp. Removing heat from such designs, using ambient air currents, is difficult since the lamp may be mounted in any orientation. Metal fins or heavy metal heat sinks are common ways to remove heat from such lamps, but such heat sinks add significant cost and have other drawbacks. It is common for such LED replacement lamps to cost $30-$60. Additionally, the light emission produced by such a solid state bulb is unlike that of an incandescent bulb since all the LEDs are mounted on a relatively small flat substrate. This departure from the standard spherical distribution patterns for conventional lamps that are replaced with LED replacement lamps is of particularly concern to the industry and end users, since their existing luminaires are often adapted to spherical light emission patterns. When presented with the typical “hemi-spherical” type emission from many standard LED replacement lamps, there are often annoying shadow lines in shades and strong variations in up/down flux ratios which can affect the proper photometric distributions.
What is needed is a new approach for a solid state lamp that replaces a variety of standard incandescent lamps, using standard electrical connectors and supply voltages, where removing adequate heat does not require significant cost or added weight and where other drawbacks of prior art solid state lamps are overcome.